


Special Cake

by PineTrain



Category: Gravity Falls
Genre: Birthday, Birthday Sex, F/M, Incest, pinecest - Freeform
Language: English
Status: Completed
Published: 2017-09-01
Updated: 2017-09-01
Packaged: 2018-12-22 09:38:00
Rating: Explicit
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 1,594
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/11964705
Author URL: https://archiveofourown.org/users/PineTrain/pseuds/PineTrain
Summary: Inspired by http://pinecestsketching.tumblr.com/post/164832972648





	Special Cake

**Author's Note:**

> Inspired by http://pinecestsketching.tumblr.com/post/164832972648

“Hey, Dipper, can you, uh…”

“Nopesorry!” Dipper said quickly, his words melding together while he shoved his phone in his pocket. He swept an arm over his desk to push his various belongings into the gaping maw of his backpack and started swiftly walking to the door of the room. Whatever question had been about to be asked didn't matter to him in the slightest.

He would have cancelled his weekly tutoring session if it hadn't been for Mabel insisting he not. It was a baffling demand, to say the least, and a somewhat irritating one at that. It was their birthday, why did she want him to be away?

While they kept their relationship secret back home, here in Gravity Falls they felt free to express their own odd form of love. He’d wanted their 18th to be just a nice, quiet, full-day date until the inevitable surprise party at the end where everything went wild with friends and family. But no, he gets punted out of the Shack around noon and just has to occupy himself until he gets a text to come back. That was hours before his tutoring session and now she'd texted him only about 20 minutes in. What even was the point?

Dipper grumbled under his breath as he walked quickly in the direction of the Shack. He knew Mabel would only do this if she'd planned something, so his irritation was mixed with a healthy bit of excitement. She hadn't let him return the favor after waking him up with a birthday blowjob and Mabel wasn't in the habit of giving sexual favors without expecting something in return. Or, she actually was, she just always made sure to remind him of that “fact” when she did want something in return. He didn't really know when he'd have a chance to do so, though, since he fully expected to find the Shack packed.

And so his interest was piqued when he reached the turn in the road that should have revealed an illuminated Mystery Shack. There were no cars out front, and no sound eminated from the building as he approached. Either the citizenry of Gravity Falls had been dumping their skill points in stealth since their last failed surprise or none of them were here. The plot thickened.

Dipper’s pace slowed as he closed in. A tingle in his spine told him something had gone wrong, but he mentally slapped it away. This was Mabel’s doing, he knew it. He wasn't sure what she had planned, so he stayed wary, eyes flicking about in search of a mistake on her part. He knew she knew he'd be doing that, but competition with each other at times like this was paramount in keeping things interesting when you've literally known your lover as long as you've known yourself.

A faint blue glow was visible through the screen door as he came closer. He entered the building and saw it was coming from the direction of the staircase. Even as he moved towards the source of the light, it never seemed to grow brighter. Some sort of lure magic, no doubt. At a distance he hadn't seen any light coming from attic window, yet he knew that was where he’d find the source.

Indeed, right when he opened the door to their room, everything went dark. Dipper’s instincts tried to force him into a pose better suited for fight or flight, but he knew his sister and ignored them. Crossing his arms, he merely waited in blackness.

Light exploded everywhere as floodlights turned on. Dipper’s vision was a mess at the change, but it didn't hurt (probably more magic light) so he maintained his stoic posture. An enormous cake lay before him, which explained why Mabel had needed him away for so many hours. It three enormous layers tall and adorned with pine trees on the middle layer and stars on the bottom. Rainbow ribbons connected each instance of decoration to its fellows; a nice match to the rainbow flames of the candles that sat atop the middle layer. Since they ran the full circle, Dipper couldn't see them all, but he knew there were 18. The only reason they wouldn't be on top was…

“HAPPY BIRTHDAY!” Mabel cried, launching herself up and out the top of cake.

Dipper grinned widely as she tossed her arms out dramatically and squinted her eyes in glee. He loved that energy of hers and the toothy grin covering her face was impossibly infectious. It wasn't the surprise he'd expected, but damn if he didn't love it.

His own grin, though, broke as his eyes naturally drifted downwards and he saw she was completely topless except for two star pasties covering her breasts. He nibbled his lower lip as he understood why their friends and family were absent tonight. Whatever Mabel had done to earn them this night alone he would figure out later, for now his eyes drifted back up to meet hers as she dropped her hands to her hips.

“See something you like, Sir Dippingsauce?”

“Oh yes, Lady Mabelton.”

Mabel laughed and placed her hands on the edge of the tube she stood in. Raising herself up with her arms, she lifted her legs over the edge and set them on the cake. Letting go, she slid down to the floor and bounced once before landing. Turning to look at her nude backside, she raised a hand to her mouth.

“Oh no! My butt and legs got all messy!” she exclaimed. “If only someone could help clean me up.” She folded her arms and looked aside miserably.

“Happy to oblige, ma’am,” Dippeer said, bowing as he feigned taking off a tophat.

“Oooh, such a gentleman. And in this day and age?” Mabel giggled.

“We do still exist, ma’am,” Dipper said. “If you would?” he gestured towards their bed.

Mabel strutted past him and leaned over the edge of the mattress. She spread her legs, revealing her moist pussy, but Dipper had to have dessert before the main course in this particular meal.

He knelt between her legs. She'd kept her feet up when she slide down the cake, so frosting was only on her upper thighs. He started with her right leg, placing his tongue at the lowest bit of sugary goodness and dragging it slowly upwards until he’d collected enough that he had to swallow. He felt Mabel shiver under his fingers as they followed on the clean parts of her leg.

He did it twice more on her right leg, before switching to her left. However, when he reached the highest point on the first lick this time, he let his right hand keep moving up and slipped a finger inside her pussy. He rubbed inside her steadily as his tongue made its gradual way up her leg, Mabel uttering soft moans all the while.

Dipper might have actually tried to fully cleaned her backside if he had anything like his sisters sugar tolerance, but six licks was enough for him. Enough for her too, he could tell by the way she wiggled her hips. He was only teasing her at this point, so he spread her cheeks and gave her lower lips a kiss, then stood up.

Unfastening his pants, he leaned over her and whispered in her ear, “Happy birthday. I love you.”

Mabel turned her head back and nuzzled her nose to his. “I love you too,” she whispered back.

Dipper pulled his hips back as he removed his member from its confines. He took hold of his shaft and guided the head to her entrance, lubing himself with small circles that spiralled closer and closer until he slipped in.

They gasped together at the connection. It felt good. Very good. Almost like the first time they made love, except even better.

“M-mabel?” Dipper asked, surprised.

“H-heh! I m-made a deal, hee! With the gn-oh-gnomes fffffor, oh-god, uh? Powdered shhhhugggar, oh, keep going!”

Dipper was only half listening as she struggled to explain. The other half of his attention was focused on fucking her harder than he ever had before. She felt amazing. So unbelievably, overwhelmingly amazing he couldn't help himself but want more of her.

He understood the gist of what she was getting at. Spiked frosting and cake with some kind of magical stimulant. Sneaky little girl. He figured she need some punishment and scooped a glob from her buttcheek then slipped it into her mouth. She happily ate it, squealing with pleasure as her pussy clamped down on him and she sucked greedily on his fingers.

Dipper laughed wildly at her reaction, before drawing down into a deep groan as her tight walls proved too much for him to resist. He thrust hard inside her as he came, hips bucking in small, frustrated attempts to be deeper. “Fucking hell, Mabel,” he hissed through gritted teeth.

They collapsed on the bed, but only momentarily. Dipper rolled off of her, feeling more energy than he normally would after such a session. He eyed his sister as she rose.

She grinned and strode over to the cake. He noticed a line of cum track down her leg as she reached it.

“I got the good stuff, Dip,” she said, swiping a finger through the frosting. She turned to face him, then bent over and caught the trickle of cum on the treat. Bringing it to her mouth she ate it with a glimmer in her eyes.

“Tonight's gonna be fun,” they said at the same time.


End file.
